2) Beat the butter and the sugars with an electric hand mixer on medium-high for about 2 minutes until light and fluffy.
3) Scrape down the sides of the bowl. Add the egg yolk and vanilla and beat on medium speed for about 20 seconds, until just incorporated.
4) Add the treacle and ginger preserves and continue beating for 30 seconds or more, until the batter is an even light brown color.
5) Add the dry ingredients all at once, beating slowly to make a soft, smooth dough. Use a rubber spatula to make sure all ingredients are combined. Then beat again for 20 seconds. Cover the bowl with cling film and refrigerate until the dough is firm, about 25 minutes.
6) Put about 110g sugar in a small bowl. Then, use a cookie scoop or a small ice cream scoop to portion the dough into a slightly heaped tablespoon for each cookie.
7) Roll the dough, by hand, into balls. Roll the tops of the balls in the sugar, and space them 5cm apart on a nonstick or lightly oiled baking sheet. Refrigerate for about 25 minutes until firm. (The chilling is what gives this cookie a beautiful, crackly crunch on top, and a soft, chewy center.)
7) Preheat oven to 190C/Gas 5. Bake on centre shelf for about 15-20 minutes until the top is crackly, and the insides peeking out are dark and moist but not raw.
8) Briefly cool the cookies on the baking sheets, then transfer to racks to cool completely. May be store in a tightly sealed container for up to 3 weeks.
Cook's Note: Ginger preserves have finely chopped ginger pieces that provide lots of flavour without hard chunks. It can be found in most supermarkets.